[PATCH] drm/msm: Set dma maximum segment size for mdss

[Sean Paul]

From: Sean Paul <seanpaul at chromium.org>

Turning on CONFIG_DMA_API_DEBUG_SG results in the following error:

[   12.078665] msm ae00000.mdss: DMA-API: mapping sg segment longer than device claims to support [len=3526656] [max=65536]
[   12.089870] WARNING: CPU: 6 PID: 334 at /mnt/host/source/src/third_party/kernel/v4.19/kernel/dma/debug.c:1301 debug_dma_map_sg+0x1dc/0x318
[   12.102655] Modules linked in: joydev
[   12.106442] CPU: 6 PID: 334 Comm: frecon Not tainted 4.19.0 #2
[   12.112450] Hardware name: Google Cheza (rev3+) (DT)
[   12.117566] pstate: 60400009 (nZCv daif +PAN -UAO)
[   12.122506] pc : debug_dma_map_sg+0x1dc/0x318
[   12.126995] lr : debug_dma_map_sg+0x1dc/0x318
[   12.131487] sp : ffffff800cc3ba80
[   12.134913] x29: ffffff800cc3ba80 x28: 0000000000000000
[   12.140395] x27: 0000000000000004 x26: 0000000000000004
[   12.145868] x25: ffffff8008e55b18 x24: 0000000000000000
[   12.151337] x23: 00000000ffffffff x22: ffffff800921c000
[   12.156809] x21: ffffffc0fa75b080 x20: ffffffc0f7195090
[   12.162280] x19: ffffffc0f1c53280 x18: 0000000000000000
[   12.167749] x17: 0000000000000000 x16: 0000000000000000
[   12.173218] x15: 0000000000000000 x14: 0720072007200720
[   12.178689] x13: 0720072007200720 x12: 0720072007200720
[   12.184161] x11: 0720072007200720 x10: 0720072007200720
[   12.189641] x9 : ffffffc0f1fc6b60 x8 : 0000000000000000
[   12.195110] x7 : ffffff8008132ce0 x6 : 0000000000000000
[   12.200585] x5 : 0000000000000000 x4 : ffffff8008134734
[   12.206058] x3 : ffffff800cc3b830 x2 : ffffffc0f1fc6240
[   12.211532] x1 : 25045a74f48a7400 x0 : 25045a74f48a7400
[   12.217006] Call trace:
[   12.219535]  debug_dma_map_sg+0x1dc/0x318
[   12.223671]  get_pages+0x19c/0x20c
[   12.227177]  msm_gem_fault+0x64/0xfc
[   12.230874]  __do_fault+0x3c/0x140
[   12.234383]  __handle_mm_fault+0x70c/0xdb8
[   12.238603]  handle_mm_fault+0xac/0xc4
[   12.242473]  do_page_fault+0x1bc/0x3d4
[   12.246342]  do_translation_fault+0x54/0x88
[   12.250652]  do_mem_abort+0x60/0xf0
[   12.254250]  el0_da+0x20/0x24
[   12.257317] irq event stamp: 67260
[   12.260828] hardirqs last  enabled at (67259): [<ffffff8008132d0c>] console_unlock+0x214/0x608
[   12.269693] hardirqs last disabled at (67260): [<ffffff8008080e0c>] do_debug_exception+0x5c/0x178
[   12.278820] softirqs last  enabled at (67256): [<ffffff8008081664>] __do_softirq+0x4d4/0x520
[   12.287510] softirqs last disabled at (67249): [<ffffff80080be574>] irq_exit+0xa8/0x100
[   12.295742] ---[ end trace e63cfc40c313ffab ]---

The root of the problem is that the default segment size for sgt is
(UINT_MAX & PAGE_MASK), and the default segment size for device dma is
64K. As such, if you compare the 2, you would deduce that the sg segment
will overflow the device's capacity. In reality, the hardware can
accommodate the larger sg segments, it's just not initializing its max
segment properly. This patch initializes the max segment size for the
mdss device, which gets rid of that pesky warning.

Reported-by: Stephen Boyd <swboyd at chromium.org>
Signed-off-by: Sean Paul <seanpaul at chromium.org>
---
 drivers/gpu/drm/msm/msm_drv.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/gpu/drm/msm/msm_drv.c b/drivers/gpu/drm/msm/msm_drv.c
index 9c9f7ff6960b..ea74542d08e4 100644
--- a/drivers/gpu/drm/msm/msm_drv.c
+++ b/drivers/gpu/drm/msm/msm_drv.c
@@ -492,6 +492,14 @@ static int msm_drm_init(struct device *dev, struct drm_driver *drv)
 	if (ret)
 		goto err_msm_uninit;
 
+	if (!dev->dma_parms) {
+		dev->dma_parms = devm_kzalloc(dev, sizeof(*dev->dma_parms),
+					      GFP_KERNEL);
+		if (!dev->dma_parms)
+			return -ENOMEM;
+	}
+	dma_set_max_seg_size(dev, DMA_BIT_MASK(32));
+
 	msm_gem_shrinker_init(ddev);
 
 	switch (get_mdp_ver(pdev)) {
-- 
Sean Paul, Software Engineer, Google / Chromium OS